From PHP a Simple Linux Script to return "Hello World"

Okay, happy to accept down votes to get to the bottom of my problem. I am brand new to Linux but have hosted my PHP based site on a Linux cloud server and need to run a FFMPEG script to convert a video. Under windows I have the full application working but under Linux I can't seem to get a basic script working and was wondering if someone can point me in the right direction. I have read the myriad posts on this, for example, Run bash Command from PHP but it does not help me get to the bottom of it.